Recently, photocatalystic activities of some of semiconductor materials, such as titanium oxide (TiO2), which exhibit an oxidation decomposition function, an antibacterial action, and an antifouling function, have been attracted attentions. In the semiconductor material having the photocatalystic activities as above, typically, electrons present on the valence band are sifted to the conduction band, as light having the energy corresponding to a band gap between the valence band and the conduction band is absorbed. The electrons sifted to the conduction band have characteristics that the electrons move to a material adsorbed on a surface of the semiconductor material having the photocatalystic activities. In the case where a material is adsorbed on a surface of the semiconductor material, therefore, the material is reduced with the electrons. Meanwhile, holes are generated on the valence band, as electrons presented on the valence hand are sifted to the conduction band. Then, the holes generated on the valence band have characteristics that the holes take electrons out from a material adsorbed on a surface of the semiconductor material having photocatalystic activities. In the case where a material is adsorbed on a surface of the semiconductor material, the holes take electrons out of the material to thereby oxidize the material.
The aforementioned phenomenon is specifically explained. Taking titanium oxide having particularly excellent photocatalystic activities as an example, electrons present on a valence band of the titanium oxide are sifted to a conduction band thereof, once titanium oxide absorbs light having energy corresponding to a band gap between the balance band and conduction band thereof. The electrons sifted to the conduction band reduce oxygen in the air to generate super oxide anions (.O2−). Meanwhile, holes are generated on the valence band as a result of the shift of the electrons. The holes generated on the valence band oxidize water adsorbed on a surface of the titanium oxide, to generate hydroxyl radicals (.OH). Since the hydroxyl radicals have extremely strong oxidizability, in the case where organic matter is adsorbed on the surface of the titanium oxide, the organic matter is decomposed by the function of the hydroxyl radicals. Ultimately, the organic matter is decomposed down to water and carbon dioxide. When light having energy corresponding to a band gap of a semiconductor material between a valence band thereof and a conduction band thereof is applied to the semiconductor material having photocatalyst activities, such as titanium oxide, as mentioned above, the semiconductor material absorbs the light, and organic matter adsorbed on a surface of the semiconductor material is decomposed. As a result, an oxide decomposition function, antibacterial action, and an antifouling function are exhibited.
Therefore, the semiconductor material having photocatalyst activities, including especially titanium oxide, has been recently widely used, as an antibacterial agent, sterilizer, antifouling agent, deodorant, or environmental cleaning agent. For example, disclosed is to provide antibacterial activities to a press bottom of electronic equipment with adhering photocatalystic titanium oxide onto the press bottom (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-195345). Moreover, disclosed are a photocatalyst thin film containing particles having photocatalystic action, which is formed of a metal element that has an electronegativity smaller than 1.6, is an element having an ion radius smaller than 0.2 nm, and has an atomic value of 2 or smaller, and a product having the photocatalyst thin film on a surface of a base (see, for example, JP-A No. 2003-305371).
However, those disclosed above have the following problems. Light energy necessary to cause excitation of titanium oxide having excellent photocatalyst activities is 3.2 eV to 3.3 eV. As this light energy is converted into a wavelength of light, the wavelength is about 380 nm. This means that the titanium oxide cause excitation when near ultraviolet rays are applied, but the titanium oxide does not cause excitation when visible light (wavelength: 400 nm to 800 nm) is applied. A proportion of ultraviolet rays in sun light is small, i.e., just 4% to 5%. In the case where sun light is used as irradiation light, therefore, there is a problem that the titanium oxide does not exhibit sufficient photocatalyst activities. In the case where light emitted from an indoor florescent lamp, in which ultraviolet rays barely exist, is applied, moreover, there is a problem that the titanium oxide hardly exhibits photocatalyst activities.
Strongly desired is a development of titanium oxide, which can solve the aforementioned problem that sufficient photocatalyst activities cannot be provided to a product used under sun light or an indoor fluorescent lamp, and which can exhibit sufficient photocatalyst activities when visible light occupying 45% of sun light, and majority of light emitted from a fluorescent lamp is applied. Therefore, researches associated with response of the titanium oxide to visible light have been widely conducted.
As one example of the aforementioned researches, proposed are, for the purpose of providing visible light responsibility to the titanium oxide, a method where oxygen defects are formed in the titanium oxide, and a method where the titanium oxide is doped with nitrogen. In these cases, however, practically satisfactory results are not attained, and it is a current situation that they have remained within a research level.
Meanwhile, the titanium oxide has poor adsorption ability to an organic material. Therefore, it is desired to improve the adsorption ability of the titanium oxide against a decomposition target, in order to exhibit an oxidation decomposition function, an antibacterial action, and an antifouling function based on the photocatalystic activities of the titanium oxide.
As for a material having an excellent adsorption ability to a decomposition target, therefore, researches and developments of technologies using properties of apatite, such as calcium hydroxyapatite have been conducted, because apatite, such as calcium hydroxyapatite Ca10(PO4)6(OH)2, which is a main component of bio hard tissue, such as teeth, and bone, facilitates ion exchange with various cations and anions, has high biocompatibility and adsorption ability, and has a significant adsorption ability to organic matter, such as protein.
As for one example of the aforementioned researches and developments, disclosed is a product, in which a semiconductor material, such as titanium oxide, and a calcium phosphate-based compound, such as calcium hydroxyapatite, are combined to thereby effectively bring out characteristics of the both materials (see, for example, JP-A Nos. 2003-80078 and 2003-321313). Moreover, disclosed is calcium⋅titanium hydroxyapatite Ca9(8)Ti(PO4)6(OH)2 having a photocatalyst function, so-called photocatalyst titanium hydroxyapatite(Ti—CaHAP), which is formed by exchanging part of calcium ions in the apatite with titanium ions (see, for example, JP-A Nos. 2000-327315, 2001-302220, 2003-175338, and 2003-334883).
Even in the aforementioned photocatalyst titanium hydroxyapatite (Ti—CaHAP), however, there is the aforementioned problem that the titanium oxide hardly exhibits photocatalystic activities when light emitted from an indoor fluorescent lamp, in which ultraviolet rays barely exist, is applied.
Therefore, disclosed as a photocatalyst, which exhibits excellent absorbance to ultraviolet rays and visible light, exhibits photocatalystic activities to light of a wide wavelength range over a long period, has excellent an adsorption ability to a decomposition target, and can exhibit an oxide decomposition function, an antibacterial action, and an antifouling function, is a Ti—CaHAP photocatalyst, in which chromium (Cr) and/or nickel (Ni), and tungsten (W) and/or vanadium (V) are introduced by doping (see, for example, JP-A No. 2006-239514).
However, the disclosed photocatalyst contains an element, which may be turned into ions harmful to the environment, such as chromium (Cr), and has a problem on practical use.
Note that, in the literature above, as a metal atom constituting apatite, possible use of a metal atom, such as aluminum (Al), or lanthanum (La) is mentioned, other than calcium (Ca). As for a metal atom that can function as a center of a photocatalyst, possible use of zinc (Zn) is mentioned, other than titanium (Ti). Moreover, chromium (Cr), and nickel (Ni) are disclosed as a visible light absorbing metal atom used for doping, and only chromium (Cr), and nickel (Ni) are mentioned as a metal atom to be introduced into titanium hydroxyapatite (Ti-HAP) by doping for providing the photocatalyst with photocatalystic activities to light of a wide wavelength range, especially light including visible light. Possible use of other metal atoms has not been studied or suggested at all.
Accordingly, a current situation is that it is desired to provide a high performance photocatalyst, which does not contain a metal atom harmful for the environment, and has excellent photocatalystic activities, and to provide a production method of the photocatalyst.